This invention relates generally to radio frequency networks which include field effect transistors and more particularly to radio frequency networks which include field effect transistors having a plurality of electrically interconnected cells.
As is known in the art, field effect transistors (FET) have been used in a wide range of applications. In many microwave applications it is sometimes desired to form a microwave transmission line and a field effect transistor on a common, semi-insulating gallium arsenide substrate. Generally, the microwave transmission line is a microstrip transmission line with the strip conductor and ground plane formed on upper and lower surfaces, respectively, of the semi-insulation substrate, such substrate providing the dielectric for the microstrip transmission line. To form the field effect transistor an active layer of gallium arsenide semiconductor material is generally formed over the upper surface of the substrate. This active layer may be formed epitaxially or by ion implantation techniques. When formed using epitaxy, a doped single crystal semiconductor layer is typically deposited on the semi-insulating substrate, usually with an intervening high resistivity epitaxial buffer layer to screen out diffusion impurities from the substrate during the active layer growth. With ion implantation, dopant atoms are implanted directly into the surface of a semi-insulating gallium arsenide substrate.
In one application the field effect transistor is used as a switch to electrically couple or decouple microwave energy source and a load selectively in accordance with a switching signal. In such application a microstrip transmission line is formed with the strip conductor of such transmission line extending laterally across a portion of the upper surface of the substrate. One end of the strip conductor provides an input port for coupling to the microwave source and the other end of the strip conductor provides an output port for coupling to the load. The field effect transistor is formed in a region of the upper surface of the active semiconductor layer adjacent the strip conductor and intermediate the input and output ports formed by the ends of the strip conductor. More particularly, the field effect transistor is formed with, for example, its drain electrode connected to the strip conductor at a point intermediate the ends of the strip conductor. The gate electrode is fed by a switching signal source. The source electrode is connected to the ground plane of the microstrip transmission line. In such application microwave energy fed by the microwave source is either allowed to pass the load or is reflected back to the microwave source selectively in accordance with the response of the field effect transistor to the switching signal fed to the gate electrode.
In high power applications it is sometimes necessary to provide a field effect transistor having a plurality of electrically interconnected cells. Here the active layer has formed on the upper surface thereof a plurality of source electrodes and a plurality of drain electrodes, such electrodes being disposed in alternating relationship laterally across the active layer in the region where the field effect transistor is formed. Disposed between each source electrode-drain electrode pair is a gate electrode. In the switching circuit application the gate electrodes are interconnected together at a common gate pad which is fed by a switching signal source, as described. Likewise, the drain electrodes are electrically interconnected to a common drain pad which is typically connected to the strip conductor at a point intermediate the ends thereof, as described. Finally, the source electrodes are interconnected together and to the ground plane conductor of the microstrip transmission line. In one interconnecting arrangement, for example, the source electrodes are interconnected by a metallization layer which overlays or bridges, in insulating relationship, the gate and drain electrodes, such bridging metallization layer having its ends terminating in a pair of contact pads on either side of the field effect transistor region. The contact pads are connected to the ground plane through via holes passing through the substrate and the active layer. In another interconnecting arrangement each one of the source electrodes is interconnected to the ground plane conductor through its own via hole which passes through a substrate. It is further known that reactive elements of the transistor, which are not inherent to the operation of the field effect transistor, limit the performance of the circuit. For example, associated with each field effect transistor is a capacitance C.sub.ds between each drain electrode and the grounded source electrode. Likewise, there is a capacitance C.sub.gs between each gate electrodes and the grounded source electrode. When a multiple cell field effect transistor is formed, however, the total effective drain-source capacitance is the parallel addition of the respective drain-source capacitances between the individual drain electrodes and the total effective gate source capacitance is the parallel addition of the respective gate-source capacitances. Consequently, in the switching arrangement described above, during the nonconducting state of the field effect transistor, as when it is desired that the field effect transistor acts as an open circuit so as to pass microwave energy from the microwave source to the load, these capacitances significantly reduce the impedance of the field effect transistor and thereby reduce its effectiveness. While it is at least theoretically possible to reduce the effect of these parasitic capacitances, as by forming an inductive element in parallel with them to thereby form a high impedance "tank" circuit, such compensation technique is generally only effective over a relatively narrow band of frequencies. Further, when the source electrodes are interconnected using the overlay metallization layer described above, in order to provide adequate separation between the source contact pads and the strip conductor a short conductor is generally used to interconnect the drain pad and the strip conductor. This short conductor, however, forms a small transmission line section which, during the conducting state of the field effect transistor, provides a reactive, frequency dependent element, thereby limiting the effective operating frequency bandwidth of the circuit.